In known passive, or low pressure, solar water heating systems, a solar collector panel absorbs solar energy into a heat exchange liquid, normally water. For maximum solar heating, the collector panel is tilted from about 20 to 60 degrees from horizontal depending upon geographical latitude. In a passive system, by thermosiphon operation, cooler water is admitted to the bottom of the solar collector after traversing a storage tank which may or may not include a heat exchanger. Water leaves the top of the solar collector and passes through a one-way valve. This valve permits water heated in the solar panel to pass to the storage tank, but reverse circulation is prevented. Such action prevents reverse thermosiphoning of hot water from cooling the water in the storage tank in cloudy weather or at night. In other thermosiphon devices, reverse circulation may be prevented by having the storage tank above the solar collector. The heated water may pass through either the tank containing a heat exchanger or a heat exchanger in the tank.
In a "passive" system no external power is required to pump liquid between the collector and the storage tank, but for continuous liquid circulation the water level must be maintained above the highest inlet pipe from the collector into the storage tank. Thermosiphoning is the sole motive power for liquid circulation in the closed loop. If liquid is lost from the system there may be a break in this continuous circulation path. A break or separation in the loop of more than about one half inch of vertical height will prevent circulation under the weak force of thermosiphoning. In such a case, the system fails and the only indication may be a lack of heat output from the solar system.
If circulation is broken, the relatively small amount of heated water in the solar collector panel will not recirculate to the storage tank. Thus, cooler water cannot enter the bottom of the collector panel from the storage tank. Consequently the "trapped" liquid in the panel will boil. In the usual arrangement, the circulation system is vented to the atmosphere. Thus, the liquid in the panel evaporates, thereby further depleting water from the system. Continued exposure of the collector to solar radiation without adequate heat removal can lead to early deterioration and destruction of the collector. This is particularly true where the collector is formed of non-metallic materials.
In low pressure solar water heating systems, similar problems can arise. Such systems are similar to passive systems but may use a low pressure, or low volume, mechanical pump to assist or replace thermosiphon circulation. Mechanical pumps are of course subject to failure because of loss of electric power to the pump motor, pump bearing seizure, impeller stall and other electrical or mechanical problems. When forced circulation ceases, the liquid in the collector can overheat or evaporate so that damage to the collector panel results.
No adequate solutions have been proposed to such problems in solar water heating systems and particularly for such systems which desirably should last 10 to 20 years in continuous domestic or household water heating service, with little or no maintenance.